A spark plug is an electrical device that is used to ignite a fuel (such as, for example, a fuel-air mixture) in an internal combustion engine, by means of an electric spark. Spark plugs have an insulated central electrode which is connected by an insulated wire to an ignition coil, a magneto circuit, or another high voltage source. This central electrode forms a spark gap with a grounded terminal at a distal end of the spark plug. The distal end of the spark plug is positioned in a space including the fuel-air mixture. As a voltage difference develops between the central electrode and ground electrode, the conditions for generating a spark at the spark gap are created. When the voltage difference between the electrodes reaches a critical value, the gases in the spark gap become ionized. The ionized gases conduct electricity and therefore allow energy to flow across the spark gap, thereby creating a spark. Spark plugs usually require a voltage of between 10 to 30 kilovolts or more to create a spark.
With time, burnt fuel or oil (soot) deposits on and around the electrodes. These deposits may hinder the formation of a spark and proper ignition of the fuel-air mixture. At high temperatures, the soot deposits on the spark plug tip burns off, thereby allowing spark formation again. The self-cleaning or self-combusting temperature the spark plug must reach to burn off the soot deposits is about 500 to 600° C. In some applications, the electrode temperature may not reach these temperatures. Additionally, at the high temperatures necessary for soot oxidation, electrode oxidation and other corrosion mechanisms that lead to electrode wear increases. These mechanisms contribute to decreasing the expected lifetime of a spark plug.
U.S. Pat. No. 4,093,887 to Corbach et al. (the '887 patent) discloses a spark plug having a center electrode that is designed to decrease corrosion of copper under the influence of hot combustion gases. The center electrode of the '887 patent includes a cladding made of a corrosion resistant material (such as, a nickel alloy or a material based on chromium or cobalt) surrounding a matrix of copper or a copper alloy having high electrical and thermal conductivity. The matrix material has embedded therein filaments made of a corrosion resistant material (preferably, the same material as the cladding). The matrix material at the sparking tip of the center electrode is etched so that the cladding and the embedded filaments protrude from the electrode surface by about 50 to 500 microns. The corrosion resistant cladding and embedded filaments of the '887 patent are said to reduce corrosion of the electrode matrix material and maintain the spark gap thickness at a fixed level. While the center electrode of the '887 patent may decrease corrosion related wear, the center electrode of the '887 patent does not promote an increased electric field at the electrode tip.
The present disclosure is directed to spark plugs that decrease or overcome the shortcomings discussed above and/or other shortcomings in existing technology.